More precisely, the invention proposes a measuring instrument, also known as an air speed indicator (ASI) suitable for providing on-board instruments with information about the performance of the aircraft, and specifically its speed, in a manner that is as accurate as possible, in particular when the aircraft is traveling at low speed.
To do this, the invention also provides a method of processing information from said measuring instrument enabling the speed of the aircraft relative to the air to be obtained, also known as “aerodynamic speed”, “true air speed”, or indeed “flight path air speed”.
It is recalled that an ASI is generally a differential pressure gauge measuring the difference between static pressure (or ambient pressure) that is independent of speed, and the pressure that is given by an intake for total air pressure, which intake is known as a “Pitot tube” by the person skilled in the art. In principle, the static pressure intake and the total pressure intake are grouped together in a single pressure probe, also known as a “Pitot probe” or a “Pitot head”, that is of substantially streamlined and cylindrical shape, with a generally hemispherical anterior portion placed on the aircraft in such a manner that firstly the total pressure intake is situated at the extreme upstream point of the cylindrical body (known as the stagnation point when the axis of the cylinder is parallel to the incident air stream upstream from said stagnation point), and secondly the static intake is radial and behind the total pressure intake.
Thus, and in application of Bernoulli's theorem (which holds good up to about 300 kilometers per hours (km/h), beyond which Saint-Venant's equations apply, or indeed Lord Rayleigh's in the supersonic range), the above-mentioned pressure difference is equal to the dynamic pressure from which the indicated air speed Vi of the aircraft along the axis of the cylindrical body of said probe is deduced. The probe is connected to an on-board instrument, i.e. an indicator constituting an ASI such that the indicated speed corresponds:                on the ground, to the speed of the aircraft relative to the surrounding atmosphere; and        in the air (in flight) to the equivalent speed, i.e. the product of the speed multiplied by the square root of the relative density of the air, itself equal to the quotient of the density of the air at the altitude in question divided by the density of air at ground level in a “standard atmosphere”. When the atmosphere on a particular day differs significantly from the standard atmosphere, a correction is included based on “altitude density” that need not be described herein.        
However, in fact, the indicated speed Vi differs from the equivalent speed because the instrument is not made perfectly (instrumental error associated in particular with making the static intake) and because the static intake does not give exactly the static pressure at infinity as would be appropriate. The static pressure is the pressure that would be measured by a probe without any speed relative to the air, and it is also necessary to distinguish local static pressures (a function of the flow of air around the aircraft or pressure field) and the static pressure of the surrounding air in the absence of any interaction, known as the “static pressure at infinity”, it being understood that this is the pressure that ought to be measured by the ASI installation.
It can thus readily be understood that developing and using an ASI is difficult, requiring special corrections and calibrations.
Furthermore, it is important to observe that traditional devices (ASI with Pitot tube and static pressure intake) have sensitivity that tends to become zero when the speed of the aircraft relative to the air becomes small.
Furthermore, and as mentioned above, the calibration of such devices leads to a single speed value being given, i.e. the speed along the axis of the cylinder of the Pitot probe. Consequently, the speed of the aircraft is assumed to be said single value: no account is taken neither of the exact value of the modulus of the air speed vector along the flight path, nor of its direction in three-dimensional space.
Finally, and in the special circumstance of rotorcraft, such aircraft can execute three possible types of flight:                vertical flight, up or down;        stationary flight or hovering, when the aircraft does not move relative to the air; and        flight in translation, whether horizontally or inclined.        
In the range of flight close to hovering, the existence of vertical and lateral components in the flight path air speed can induce angles of incidence and of side-slip between the flight path air speed vector and the rotorcraft that can reach large values, making the air speed indicated by such traditional devices badly wrong.
With rotorcraft, the wash from the rotor(s) also disturbs indications in this portion of the flight envelope.
Because of these phenomena, limitations are put on the use of rotorcraft in order to guarantee safety, with the corresponding reduction in the utilization and the performance of such aircraft.
Consequently, in order to avoid loss of sensitivity, certain helicopters are fitted with two Pitot tubes placed on two opposite arms of a rotary antenna centered on the axis of rotation of the main rotor, above the plane of the blades. That device is known as an omnidirectional air data system and is used for example on military helicopters such as the Bell UH60 helicopter or indeed an analogous device is implemented on the Dauphin™ Coast Guard helicopter developed by the Applicant.
The MI28 helicopter uses Pitot tubes placed directly at the ends of the blades.
Those two devices present sensitivity that is more or less constant, including at low speed, and they can provide the two components of air speed in the plane of rotation of said device, to the exclusion of the third component that is orthogonal to said plane of rotation.
In contrast, those devices cannot be used on civilian rotorcraft because of their complexity and their cost.
Under such conditions, U.S. Pat. No. 3,373,605 presents a device mounted on an arm that rotates at constant speed, thus constituting a rotating antenna, and fitted at each end with a total pressure intake or Pitot tube. That device enables the absolute value of relative speed to be measured in the plane of rotation of the antenna between said device and the incident air stream, and also makes it possible to measure the direction of that air stream in said plane. The pressure difference between the two total pressure intakes is a periodic function having amplitude that is proportional to the modulus of said relative speed. In practice, the relative speed is obtained directly from the maximum value of the pressure difference. Side-slip is calculated from pressure difference values measured at “points” that are spaced apart orthogonally. No information on a possible relative speed component normal to the plane of rotation of the device can be determined.
U.S. Pat. No. 3,318,146 relates to a device for measuring the speed and the angle of inclination of a flow by means of a probe having at least five orifices, acting as pressure intakes and placed on the surface of a sphere in a special manner. As a result, that device enables the Mach number of the flow to be obtained together with information concerning the orientation of the speed vector, in particular by making use of experimental curves. Consequently, that equipment cannot satisfy the need for an aircraft air speed indicator, in particular when the aircraft is a rotorcraft flying at low speed. It is recalled that Mach number corresponds to the speed of a vehicle divided by the speed of sound. The speed of sound is proportional to the square root of the absolute temperature of a fluid, specifically the air at the probe, so the use of the device in question makes it necessary, at least in particular, to use an additional probe to measure temperature and appropriate processor means. Such a device is necessarily expensive and not sufficiently reliable because of the coupling between the above-mentioned means and the use of empirical results (experimental curves). In addition, the sensitivity of that device tends towards zero as the speed of the aircraft decreases.
In an attempt to provide a solution to the problem of obtaining the speed of a helicopter, document FR-2 282 644 describes a device for determining speed components in a plane parallel to the rotor plane during flying stages close to hovering conditions. That device performs said function via two measurement systems associated with the pitch cyclic control system of the rotor under the control of a control stick, having a first detector for detecting the position of the rotor pitch cyclic control along the transverse axis, and a second detector for detecting the component of acceleration along the longitudinal axis, together with calculation means, associated with said detectors, to integrate the algebraic sum of the values measured by the two corresponding detectors relative to each of the above-mentioned axes.
Document FR-2 565 270 relates to an improvement of the above document that differs by the fact that it includes, in each of the two measurement systems, a single detector (specifically an accelerometer) for detecting the position of the rotor pitch cyclic control relative to the axis in question and the acceleration component along the same axis, together with calculation means serving to integrate the output signal from the single detector of each measurement system so as to provide the speed components of the helicopter relative to air and relative to the axes in question.
The devices disclosed in the two above documents do not enable all three components of the flight path air speed vector to be obtained, but only the longitudinal and transverse components.
Document FR-2 688 314 describes determining the two components of the air speed in the plane of the rotor in accordance with document FR-2 565 270, and teaches certain novel dispositions that enable the vertical component of the speed of the helicopter relative to the surrounding mass of air to be determined when said speed is small, with this being done solely by using internal measurement means, i.e. the position of the lift rotor controls (using accelerometers), the speed of rotation of the rotor, and the three components of acceleration in the cabin. Those dispositions are applied to certain mass-produced helicopters and do indeed provide information that is useful for the pilot and that is used by on-board systems, but nevertheless with accuracy that is limited by the fact that the accuracy of the pendular system (according to document FR-2 282 644 only) and the actual principles used for determining speed (see documents FR-2 282 644, FR-2 565 270, and FR-2 688 314): direct aerodynamic measurement is not involved, rather indirect identification via the response of the aircraft to said speed based on a very rudimentary mechanical model of helicopter flight. Even if the model takes account of changes in the configuration of the helicopter, it ignores the influence of numerous parameters such as the non-uniformity of the flow of air around the rotor, the number of blades, the shape of the blades, the kinematics of the hub, . . . . Those simplifying assumptions lead to errors concerning the calculated values of the speed components, which errors can be cumulative and can become very large, thereby limiting the representivity of the results. Furthermore, that device (see document FR-2 282 644) requires the zero setting system to be calibrated frequently, since otherwise there is a danger of losing accuracy at low speed. Consequently, that type of system does not make it possible to do without an ASI of the usual type, since it cannot be calibrated for high speeds, and it does not make it possible to cover the entire flight envelope of the aircraft. Thus, none of the known dispositions provides a satisfactory solution to the need.